A modular re-usable buoyancy and flotation apparatus having conveniently connected buoyancy elements to form buoyant arrays of desired sizes, shapes and depth/pressure ratings is adapted for varied jobs in marine construction, scientific research and other subsea applications. The apparatus includes a skeleton of linear supports traversing buoyant shapes, the supports terminating in inter-element connection fixtures that extend and retract resiliently and can compensate for compression of the buoyant medium at the hydrostatic pressures of different undersea depths.
It is known to attach one or more buoyant elements to weight (a “load”) that is to be manipulated below a water surface, e.g., in the sea. Up-thrust from the buoyant element supports some of the weight of the load. The attached load and buoyant element are a movable structural unit, with at least a reduced need to support the weight of the load by other means. Conversely, a load might be buoyant, in which case one might choose to attach one or more weights to offset the up-thrust of the buoyant load.
The present disclosure seeks to provide a practical way to enable buoyant elements to be attached to weighted loads to accommodate loads in various configurations. The configurations can involve loads with distributed higher and lower density, i.e., concentrated weights and buoyant volumes at different places, causing structural stresses. These are handled by structurally attaching versatile modular buoyant elements in arrays. Provisions are made to enable deployment of over a considerable range of depth and pressure, namely by grading the buoyant elements in depth ranges using syntactic flotation materials, to bear compression forces. Inasmuch as the buoyant elements are modular, they are aptly recoverable for re-use, and can be reconfigured in wholly new combinations, capacities, shapes and arrays.
Each modular flotation elements provides an incremental amount of buoyancy. A combined and preferably positively-attached set of low cost buoyancy arrangements offset at least part of the weight of subsea deployed units, instruments, equipment and vehicles of various sizes, configurations and weight-versus-buoyancy distribution.
Some or all of the flotation elements (or groupings of elements) employ syntactic flotation materials containing water-pressure resistant bodies. Sealed hollow beads or similar shapes that are small, and individually structured to withstand expected pressure without substantial compression, can be distributed evenly in a carrier medium. The forces associated with buoyancy are such that that the up-thrust provided by a buoyant element is equal to the weight of the column of water that the element displaces. The volume of a body of buoyant material often decreases with increased water pressure. Use of syntactic flotation materials tends to reduce the compression, but it is necessary to configure the syntactic flotation material to withstand expected pressures.
The flotation modules can be designed as regular polyhedral shapes such as cubes of various sizes, preferably for selected depth ratings. These shapes are arranged to abut, stack and nest with one another. In certain examples the flotation modules are equally sized and abut or stack as blocks. Alternatively, larger and smaller elements can be combined, e.g., to fill out volumes that are partly occupied, such as by parts of the load or its carrier.
The modules can integrally include or structurally receive and engage with restraint and structural mounting components by which the modules forming the components of an array are fixed relative to one another and attached to supported loads. The forces associated with dynamic motion during launch and or recovery through the air/water interface, as well as buoyancy in water, are accommodated in exemplary embodiments through a system of interlocking restraint members that engage in self-supporting manner with integral strength members incorporated in the modules or used to couple the interlocking restraint members. The modular system is scalable, accommodating small installations, for example with 2 to 10 modules or block assemblies of plural modules, up to large installations of connected flotation block assemblies or concurrently-deployed independent flotation block assemblies that are tethered to a load and optionally to one another by the available interconnect and mounting hardware. Moreover, the structures employed for interlocking assembly conveniently permit an assembly of the flotation blocks and assemblies to be configured in regular or irregular shapes, advantageously to complement the weight distribution of the supported load.
In the marine construction and scientific industries, manned or unmanned vehicles (craft), remotely operated or autonomous vehicles and the like, are useful to guide or carry various functional and structural items to locations between the surface and the seafloor at various depths. The items as well as the craft can be regarded as payload items to be delivered and recovered. The payload items may be functional units, structural parts to be assembled and temporarily or permanently installed. The payload items may include intervention tools or instruments. Any of various possible payloads can perform a myriad of tasks at any point from the ocean surface to the seafloor. A challenge for operators of vehicles and divers or on-board vehicle crew is to maintain a manageable near neutral buoyancy when attempting to move a payload in seawater, so that the vehicle can control the depth and position of the payload during transport and also during manipulation at the subsea work site, without the need for the vehicle to exert a great deal of thrust. Advantageously, the present disclosure assists in this process while avoiding or minimizing the need for complications such as ballast tanks or other means to alter buoyancy by enabling the vehicle to assume or shed weight.
A subsea transport unit or vehicle can be used like a farm tractor deploying a farm implement. The subsea transport unit or vehicle provides motive force, vertical and horizontal thrust, local control and electric, hydraulic or other power take-offs to operate a subsea tool package. A basic subsea vehicle is advantageously designed with a moderate amount of reserve buoyancy to offset the expected weight of a self-mounted payload of tools and/or instruments or controllable ballast arrangement to assist in lifting the payload during recovery. Often the wet weight of the tools and instruments that are sought to be deployed is higher than the base vehicle's reserve buoyancy, plus vertical thrust. In that case, some or all of the wet weight of a payload can be offset by adding supplemental buoyancy, preferably by accommodating reconfigurable re-usable flotation modules as disclosed herein.
Deployment and installation of subsea structures, equipment or tools to a seafloor oilfield production spread is an exemplary application of this technology, wherein the flotation modules can be adapted to the particular use, thus enabling precise positioning and alignment of the payload by reducing the need to counteract weight or up-thrust, as encountered.
Steel structures and equipment in such applications can be heavy in water compared to a transport vehicle, and sometimes expensive heavy lift cranes are deployed to lower and position heavy payloads from the surface. However, preferably internally mounted flotation arrangements designed to be incorporated into equipment tethered or included as part of a subsea vehicle, reduce the in-water weight of the payload and/or subsea vehicle so that the vehicle can readily align and position the payload using modest thrusters. Permanent installation of flotation on the equipment is not always desirable or cost effective. The equipment might be intended to be mounted and to remain subsea, in which case the flotation arrangements are superfluous. If the tool package is to be used or to be variably configured from mission to mission, the flotation arrangements for one mission may not be apt for another mission.